Summary:
IlvI is the large catalytic subunit of the bifunctional acetohydroxybutanoate synthase /acetolactate synthase (IlvI/H, AHAS III) which carries out both the first step in valine biosynthesis and the second step in isoleucine biosynthesis. The IlvI/H protein complex catalyzes the conversion of pyruvate and oxobutanoate into 2-aceto-2-hydroxy-butyrate and the conversion of pyruvate into 2-acetolactate. Both reactions generate carbon dioxide as a product. [De74, Gollop89, Barak87, Vyazmensky96, Weinstock92]

C-terminal and N-terminal ilvH mutants were constructed and used to determine the minimum activation peptide necessary to activate IlvI [Zhao13]. Interactions between large and small subunits of different AHAS isozymes were investigated. IlvI could be activated by IlvM just as well as IlvH. [Vyazmensky09]

Isolated IlvI has only 5% of the molar activity of its holoenzyme. However, isolated IlvI has similar substrate specificity and similar cofactor dependence as its holoenzyme. Assembly of the holoenzyme requires FAD. [Vyazmensky96]

Summary:
Acetohydroxy acid synthase III (AHAS III) is one of two functional isozymes catalyzing the decarboxylation of pyruvate and transfer of the resulting acetaldehyde group to either pyruvate or α-ketobutyrate, producing α-acetolactate for the valine pathway and α-aceto-α-hydroxybutyrate for the isoleucine pathway. This is the first common step in the biosynthesis of the branched-chain amino acids isoleucine, leucine, and valine. A third isozyme, AHAS II, is not functional in E. coli K-12 due to the presence of a frame shift mutation in the gene encoding the large subunit, ilvG. [Neidhardt96]

In the presence of both pyruvate and α-ketobutyrate, AHAS III produces approximately 40-fold more acetohydroxybutyrate than acetolactate, while AHAS I shows no product preference [Barak87, Gollop89].

The differential regulation of enzymatic activity and expression of the isozymes has direct physiological consequences and has been under intense study. The end products of the branched-chain amino acid biosynthesis pathways all inhibit AHAS III activity, although inhibition by valine is most significant [Gollop89]. Both AHAS I and III are inhibited by valine [De78]. Activity of AHAS III is only partially inhibited by leucine, while AHAS I activity can be almost completely inhibited [Gollop83].

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History:
Socorro Gama-Castro on Fri Oct 29, 2004:
The start site of this gene was originally assigned solely on the basis of sequence
considerations [Blattner97
]. However, it was
changed because Lago et al [Lago85
] attested that the real start site is actually located 90 bp downstream.
The demonstration is based on protein sequencing.
10/20/97 Gene b0077 from Blattner lab Genbank (v. M52) entry merged into EcoCyc gene
EG10500; confirmed by SwissProt match.